(1) Field of the Invention
The present invention relates to an improved method and apparatus for aligning discontinuous fibers using spaced apart plates having an electrical field between them and a feeder apparatus for aligning the fibers for movement between the plates. The feeder apparatus is characterized by preferably having an elongated slot or slots which align the long axis of fibers in a horizontal plane for subsequent perpendicular alignment between the plates, thereby preventing the fibers from moving with the long axis in the vertical plane and essentially falling in an uncontrolled manner. Further, the invention relates to the manufacture of a non-woven, discontinuous fiber reinforced thermoplastic sheet with controlled fiber orientation distribution.
(2) Description of Related Art
Micro-mechanics models for composite materials predict that in discontinuous fiber composites, modulus and strength values approach that of the unidirectional continuous fiber composites, when the length of the fibers far exceed the critical fiber length and when the fibers are aligned in the direction of the load (Agarwal, B. D., et al., Analysis and Performance of Fiber Composites, John Wiley & Sons, 121-131 (1990); and Piggott, M., "Load Bearing Fibre Composites", Permagon Press, 72-79 and 83-89 (1980)). Aligned discontinuous fiber polymer composites have a clear advantage over other composite material systems with respect to overall performance and processability and are potentially well suited for lightweight structural applications. Several techniques using hydraulic, electrical, magnetic or pneumatic means have been tried in the past to orient fibers in a preferred direction while processing discontinuous fiber composites. However, these methods of making aligned discontinuous fiber composites have met with limited success because the methodologies that were developed did not bring about any significant reductions in fabrication times or costs.
There is a need for an improved method that can manufacture discontinuous fiber composites with the fibers preferentially aligned in one direction using electric fields. There is a need for a method amenable to high degree of automation and high speeds of operation, thereby reducing the cycle time needed to fabricate an oriented discontinuous fiber composite sheet or part.
Controlling the orientation of short/discontinuous fibers has been a challenge in the processing of composite materials be it in glass fiber sheet molding compound (SMC) processing, resin injection molding (RIM) preform manufacture or injection molding. With increased realization of the performance payoffs of aligned discontinuous composites, several attempts have been made to control fiber orientation. A review of literature relating to fiber alignment techniques that can be used in the fabrication of aligned discontinuous fiber composites is presented where the techniques are broadly classified into two categories viz. wet/slurry methods and dry methods. This categorization lends itself to the general conclusion that wet/slurry methods are typically slower and less flexible in controlling fiber orientation as compared to the dry methods.
In the wet methods (Kacir, L., et al., Polymer Engineering and Science, Vol. 15, p. 525, 532 (1975); Vol. 17, p. 234 (1977); Vol 18, p. 45 (1978); and Soh, S. K., Proc. 10th. Annual ASM/ESD Advanced Composites Conference & Exposition, Dearborn (1994)), the fibers are usually in a well agitated liquid suspension and a fiber mat is created by either draining the liquid or raising a filter bed through the suspension. Control of fiber orientation is limited, but can be achieved to some degree by guiding vanes or other means like electric fields when the liquid is dielectric (Knoblach, G. M., U.S. Pat. No. 5,057,253). The drawbacks in these processes is the introduction of an additional step of drying the wet mat which reduces the speed of manufacturing drastically, and secondly the fact that fiber mat preform has to be further processed by reaction injection molding or polymer sheet impregnation to result in a composite part. The mechanical performance of the final part may also be sometimes lowered due to the presence of voids entrapped during the drying of the wet fiber mat.
Dry methods ((Talbot, J. W., et al., U.S. Pat. Nos. 4,664,856 (1987); 4,113,812 (1978); and Peters, T. E., et al., U.S. Pat. No. 5,017,312 (1991)) usually rely on electric fields or pneumatic means ((Ericson, M. L., et al., Composites Science and Technology 49:121-130 (1993)) to control fiber orientation and are generally faster than the wet processes. Peters et al (U.S. Pat. No. 5,017,312) had developed the technology to manufacture oriented chopped glass fiber mats where a complicated array of electrodes are embedded at the bottom of the mat that is being formed and also above the mat to force orientation of the fibers as they descend. Lack of proper understanding of the fiber electro-dynamics resulted in a complicated orientation technique. Besides, the end product is a fiber preform which needs additional processing of liquid resin molding, before it can become a final composite part. Other patents of interest are U.S. Pat. Nos. 2,686,141 to Sawyer, 4,111,294 to Carpenter et al, 4,347,202 to Henckel et al, 4,707,231 to Berger and 5,017,312 to Peters et al. DuPont (Chang, I. Y., et al., J. Thermoplastic Composite Materials, Vol. 4, p 227-252 (1991)) introduced a long discontinuous fiber (LDF) thermoplastic composite prepreg for aerospace applications that has fibers several inches long which makes it less flexible in molding complex shapes. Moreover, the starting material is a continuous fiber impregnated prepreg. Another approach for making oriented preformed glass mat reinforced thermoplastics is by using spray-up techniques (Jander, M., Proc. 7th Annual ASM/ESD Advanced Composites Conference, Detroit (1991)). Although these techniques are fast, only stiff and long fiber bundles (1"-2") can be oriented by this technique which generally results in poor matrix impregnation and inflexible fiber preforms.
The problem in the prior art is that the fibers are randomly introduced between the E-field plates. In general, this tends to result in a large fraction of fibers which are not oriented by the electrical field, particularly when the fibers have a longitudinal long axis which is essentially vertical. There is also a problem with rebound of the fibers on the surface where they are to be deposited. The result is fibers which are not properly aligned, regardless of the method used to overcome distortions of the plate adjacent electrical field to the mat.
The widespread use of high performance continuous fiber composites is limited to a great extent due to expensive fabrication costs, while discontinuous or short fiber composites form a major share of the fiber reinforced composites for non-structural applications, due to ease in processability. To optimize the balance between performance and processability in polymer composite material systems, a novel high speed method was developed which produces aligned discontinuous fiber composites (ADF) using electric fields.